1. Field of the Invention
The present invention relates to the use of supercritical gases for material size reduction and separation. More specifically, this invention relates to swelling a material particle with a supercritical gas and then rapidly dropping the pressure so that rapid internal expansion of the supercritical gas inside the material particle cause the material to be torn apart and further reduced in particle size.
2. Description of the Related Art
In the area of reducing the size of materials from coarse pieces to fine powder, it is generally known to decrease the size of such materials to small particles. For example in the rubber recycling and reusing industry, it is generally known to reduce rubber chips, such as those obtained from the shredding of tires, to particles of irregular outline that pass through a minus 80-mesh or a minus 50-mesh or finer. Wood chips are reduced to a smaller particle size before ligand is extracted. By increasing the surface area of any given material, smaller particle sizes facilitate chemical reactivity and the ease at which a material will dissolve into various mixes.
A variety of techniques for reducing the particle size of materials from coarse materials to fine materials have been developed in the flour, paper pulp industry, paint pigment compounding industries and rubber recycling industries. For example, a variety of rubber products (e.g., natural rubber, synthetic rubber, vulcanized rubber, automotive tire scrap, etc.) may be reduced to coarse materials. Coarse materials are produced from a variety of methods including cutting, shredding, chopping, chipping, milling and grinding. Other known methods include the milling of a material between horizontal grinding stones in a horizontal grinding mill. Such milling techniques include grinding the material between opposed milling wheels, such that one wheel is fixed and the other wheel rotates relative to the fixed wheel. Such known milling techniques include pressing the two wheels against a material slurry, such that the rubber is ground to a fine state (i.e., powder) in a single pass. However, such known milling methods have the disadvantage of creating friction and introducing energy to the slurry, which may increase the temperature of the slurry. Increased slurry temperatures may cause xe2x80x9cSlash overxe2x80x9d in which the slurry becomes a largely dry material mass that inhibits grinding. Such known milling methods further have the disadvantage of not producing a uniformly fine material powder that passes through a minus 50-mesh. Such techniques also require large amounts of water, which requires large amounts of energy to evaporate or centrifuging. Addition methods for producing particles of a decreased size include cryogenic cracking of the material and the use of bead or mead mills. Moreover, smaller particle sizes are desirable, such as particles in the micron or submicron sizes.
Accordingly, it would be advantageous to have a method for converting a coarse material to a finely ground particle. It would also be advantageous to have a method for converting a coarse material to a finely ground particle without centrifuging or using large quantities or any quantity of water. It would also be advantageous to have a method to material to a decreased size that decreases soak time of the material. It would also be advantageous to have a method to convert a material to a decreased size that decreases grinding time of the material. It would also be advantageous to have a method that converts material to a decreased size carried out at a relatively low temperature of material slurry. It would also be advantageous to have a material that chemically combines with an additive that modifies the physical properties of the material.
In accordance with the present invention is provided a method for converting a material to a decreased size using supercritical gases. The method includes optionally preparing said material to make said material capable of being dispersed with supercritical gases. Such optional preparation involves reducing the material to a coarse particle size by shredding, chopping, chipping, milling or grinding.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The size of a material may be using a gas that is in a supercritical liquid state. According to one embodiment, the gas is supercritical carbon dioxide. The course material for size reduction may be any material that has been previously chopped, shredded, chipped, cut or pulverized milled ground etc. In on embodiment the material is between 2 and 200 mesh. In another embodiment the material is 60 to 150 mesh. In another embodiment the material is xe2x88x9240 to 2 mesh.
In a exemplary embodiment, the material may contain rubber. Examples of rubber include natural rubber, synthetic rubber, recycled rubbers containing polyethylene and/or polypropylene, vulcanized rubber, carbon black, waste from tire production, various polymers, various plastics, thermoplastic elastomers, thermoplastic vulcanates, polyethylene plastics, etc.
Gases with critical pressures at or below 1,100 psi are desirable because less pressure is needed to change the gas into a supercritical state. Higher pressures are disadvantageous because more energy is required, safety considerations and the equipment for generating such high pressures is more expensive. Any refrigerants with critical pressure of less than 1,100 psi are suitable for the present invention. Such refrigerants include freon and the Suva(copyright) gases listed in Table 2. Carbon dioxide is also suitable, but has a critical pressure that is higher than most refrigerants. An advantage of carbon dioxide is that the critical temperature is 31xc2x0 C.
Other materials may include wood chips, wood pulp or sawdust. It is desirable to extract lignin from such coarse wood materials to facilitate a paper making process that is better for the environment. Additional materials suitable for processing include elastomers, plastomers, agricultural materials, biological materials and forest materials, etc.
Coarse materials are introduced into a pressurizing vessel capable of pressurizing a gas at a temperature above freezing into a supercritical state. A gas or liquid is introduced into the vessel and the pressure and/or temperature of the vessel is increased so that the gas or liquid reaches supercritical conditions. In some uses it may be preferable to use a gas or liquid that has an ambient critical temperature. In other uses, the supercritical solvent of interest may have higher critical temperatures. The critical temperature preferably does not exceed 120xc2x0 C. Many refrigerants, such as most of the Suva(copyright) refrigerants, offered by DuPont, have critical temperatures of less than 120xc2x0 C. and thus can be used for particle size reduction. A listing of suitable Suva(copyright) refrigerants is found in Table 2. Examples of such gases include carbon dioxide and refrigerants such as Suva(copyright) 95. Gases that are relatively unreactive, non-toxic, and have a critical temperature of less than 120xc2x0 C. and a critical pressure of less than 1,100 psi are preferred.
The vessel into which the material for size reduction and the gas is introduced is made to operate at pressures sufficiently high to bring about a supercritical state in a gas that has a supercritical temperature of generally less than 120xc2x0 C. The pressure of the vessel must be sufficiently high so that when the pressure is rapidly released, the particles for size reduction are ripped apart by the rapidly outward expanding pressure. When gases are in a supercritical state, they will have a solvating effect on the coarse material. For example, tire chips can be exposed to supercritical gas in the vessel.
In one embodiment most impurities from the coarse material introduced into the vessel for mixing the supercritical gas have been previously removed. For example, if the material is recycled tire chips, the chips may have been previously delaminated and the materials such as metals, cords, and reinforcing materials might have been previously removed. In another embodiment, supercritical gas could be used to help separate impurities from the coarse material. For example, supercritical gas may be used in delaminating the tire chips as well as removing impurities (e.g., metals, cords, reinforcing materials, etc.) from the coarse material. A magnet may be used to remove certain metallic impurities from the coarsely ground material at any time during the processing of the material. For example, the impurities can be removed by passing the ground material through a screen that allows xc2xc-inch to ⅝-inch particles to pass through.
The present invention contemplates swelling the material whose size is to be reduced with a supercritical gas. Materials that have solubility parameters the same or similar solubility parameter of the supercritical gas are particularly easy to swell. When the present invention is used for tire recycling, soaking the coarse material in the supercritical gas, among other advantages, causes the material to swell, and can remove impurities from the material such as oils and dissolve adhesives in the material. The inventors believe that the solubility of supercritical carbon dioxide is more than that hexane and less than that of toluene. It is known that the solubility parameter of hexane is 7.3 and toluene is 8.9. Hexane can swell tire rubber but is unable to substantially swell vulcanized rubber. Toluene is better at swelling tire rubber. Therefore, because supercritical carbon dioxide has a solubility parameter close to that of toluene, supercritical carbon dioxide will readily swell rubber. Supercritical carbon dioxide is also advantageous because the small size of the carbon dioxide molecule allows it to better and more deeply penetrate the rubber. In one aspect of the present invention, the solubility parameter of a supercritical gas is advantageously used to soak a material that has similar solubility parameter, such as vulcanized rubber, or a material having at least a monomer of similar solubility parameter to that of the supercritical gas.
The solubility parameters of other gases with similar cohesive energy densities to supercritical carbon dioxide can also be substituted as a supercritical solvent in place of or with carbon dioxide. For example, Freon(copyright) TF has a solubility parameter of 7.25. It is expected that other refrigerants should have similar solubility parameters. Refrigerants, like carbon dioxide, are small molecules with no or a small dipole moment, and thus have similarly low solubility parameters. Thus, such molecules have weak van der Waals forces attracting each other and a low heat of vaporization. The solubility parameter delta (xcex4) is represented by the following equation:   δ  =            c        =                  [                                            Δ              ⁢                              xe2x80x83                            ⁢              H                        -            RT                    Vm                ]                    1        /        2            
where:
c=Cohesive energy density
xcex94=Heat of vaporization
R=Gas constant
T=Temperature
Vm=Molar volume
Solubility parameters may be found in the following references, which are hereby incorporated by reference in their entirety: Hansen, Journal of Paint Technology Vol. 39, No. 505, February 1967; Crowley, et al., Journal of Paint Technology Vol. 38, No. 496, May 1966; Solubility of Polymers, Encyclopedia of Polymer Technology, 2nd. Ed.;Patrick, Treatise on Adhesion and Adhesives Vol. 1, Marcel Dekker, 1967; J. Appl. Polym.Sci, Vol. 16, No. 339, 1961; Barton, Barton""s Handbook of Solubility Parameters and Other Cohesion Parameters CRC Press, 1991; Handbook of Polymer-Liquid Interaction Parameters CRC Press, 1990; Hansen, xe2x80x9cThe Universality of the Solubility Parameter Concept,xe2x80x9d I and E C Product Research and Development, Vol. 8, No. 1, 1969; Jean, Predicting Resin Solubilities. Columbus, Ohio: Ashland Chemical Technical Bulletin, Number 1206; and Giorgio, Solubility and Solvents for Conversation Problems. Rome: ICCROM, 1978; Samuel Sawan, Yeong-Tarng Shieh and Jan-Hon Su, xe2x80x9cEvaluation of the Interactions Between Supercritical Carbon Dioxide and Polymeric Materials,xe2x80x9d Los Alamos National Laboratory, Toxic Use Research Institute, U. of Mass., Lowell and, IBM, LA-UR-94-2341.
Once the material has been swelled, the high pressure of the vessel containing the supercritical gas is rapidly dropped so that a great pressure difference is created between the particles and the vessel. Before the pressure is dropped, the particles have been swollen with the supercritical gas at high pressure. The supercritical gas that is absorbed into particles creates a high internal pressure within the particles. When the pressure external to the particles is rapidly dropped, the particles rapidly expand outwards, causing the particles to rip apart and explode. Many of the resulting particles are in the micron or submicron size. Smaller particles in the micron or submicron size are desirable because they are at they size in which they are able to form colloids. Smaller particles are also better for filling and reinforcing. This process is also advantageous because the smaller rubber particles can be made with less oxidation and degradation.
When smaller particles are fed into the supercritical vessel, they may be fed in a continuous mode. Smaller particles that may be fed by a continuous mode are around 80 mesh or 180 microns in size. Such smaller particles may advantageously have a shorter swelling time in the supercritical reaction vessel than the larger particles. Coarser particles around 10 mesh or 2 millimeters will preferably by fed into the supercritical reaction vessel in a batch mode, with the use of valves or rotary airlocks. Such particles will have longer swelling time. The preferred temperatures and pressures for making a gas, such as carbon dioxide, into a supercritical gas are well known to those of ordinary skill in the art. For example, at a temperature above 32xc2x0 C., carbon dioxide can be forced into a supercritical state at a pressure of around 1070 psi. Additional critical temperatures and pressures are given in Table 2.
The supercritical gas may be used with a variety of additives, as known as coagents. These additives may be polar or nonpolar, organic or aqueous, and may also include surfactants. Additives may assist in the swelling of the rubber and help fractionalize the particles by reducing the cohesiveness of the particles.
According to an exemplary embodiment, a variety of water-soluble additives may be added to the supercritical carbon dioxide or upstream to the feedstock material. The additive, when used during soaking of the material, serves to decrease the swelling time of the material as compared to material soaked in the absence of the additive. According to an alternative embodiment, the additive may be a chemical that swells the rubber, but is not a tackifier, such as tetrahydrofuran (THF) or dimethyl formamide (DMF). According to other alternative embodiments, a variety of additives (as described below) may be used when soaking the material. According to one embodiment the additive used when soaking the material is a fatty amine, such as DELAMIN(trademark) commercially available from Hercules, Inc.
The size of the soaked material may be decreased in a grinding apparatus, such as a set of milling stones. According to an alternative embodiment, the size of the soaked material may be decreased by freezing the material and cracking or hammering the material into small sized particles. According to other alternative embodiments, the soaked material is shredded by a series of blades or an Archimedes screw apparatus. According to still other alternative embodiments, the size of a soaked or non-soaked material may be decreased by brute force (i.e., by two counteracting surfaces). According to an embodiment, grinding of the soaked material material is achieved using the grinding apparatus as disclosed in U.S. Pat. No. 5,238,194 issued Aug. 24, 1993 to Rouse et al. for xe2x80x9cMETHOD OF PRODUCING FINE ELASTOMERIC PARTICLESxe2x80x9d and incorporated herein by reference. The grinding apparatus expediently includes a horizontal grinding machine providing a fixed stator and a rotating rotor, on which disc-shaped grinding stones having hollow centers can be mounted. The grinding stones provide flat, opposing abrasive surfaces (i.e., flat annulus surface), and each grinding stone is opposed to the other. The abrasive surfaces provide periodically spaced openings in the annulus for introducing the materials to be ground between the closed, cooperating abrading surfaces.
An material slurry may be formed by adding a carrier to the ground material. According to an alternative embodiment, the carrier may be a gas (e.g., air) that may carry a wet or dry stream of material particles. According to other alternative embodiments, the carrier is a fluid (e.g., water). According to other alternative embodiments, the carrier may be provided through the grinding apparatus before the slurry is -provided through the grinding apparatus. According to any embodiment, a uniform set point fluid flow rate through a properly spaced set of grinding stones is determined as disclosed in U.S. Pat. No. 5,238,194 issued Aug. 24, 1993 to Rouse et al. for xe2x80x9cMETHOD OF PRODUCING FINE ELASTOMERIC PARTICLESxe2x80x9d and incorporated herein by reference.
According to an exemplary embodiment of the present invention, the additive may be added to the slurry. The additive provides for faster grinding of the slurry as compared to a slurry in which the additive is absent. According to an alternative embodiment the additive is a water miscible dispersing agent for carbon black, such as DAXADJ commercially available form R. T. Vanderbilt Company, Incorporated of Norwalk, Conn. According to a particularly preferred embodiment of 25% DAXADJ is added to an material slurry of 15% to 60% solids. According to an alternative embodiment, the additive is a surfactant, such as TRITON-XJ commercially available from Rohm and Haas Company. According to a particularly preferred embodiment, the additive is added to the slurry in the amount of about 1% to 20% based on the amount of material in the slurry, and about 10% to 15% by total weight of the material in the slurry.
The additive provides a higher yield at the first pass of grinding of the slurry as compared to a slurry in which the additive is absent. Use of the additive produces a higher quantity (i.e., more uniformly fine ground and having less shear) particle as compared to the particles produced from a slurry in which the additive is absent. A slurry having an additive yields a significant increase in throughput of ground particles of the desired smaller size, and a smaller percentage of particles passed through the mill without adequate reduction in size. This alone is beneficial, as the economic losses occasioned by the necessity of screening out insufficiently reduced particles and re-grinding them can be reduced. Further, use of the additive provides decreased grinding times of the slurry as compared to a slurry in which the additive is absent. Decreased grinding times may reduce wear and damage to the wheels of the grinding apparatus.
The additives provide a lower temperature to the slurry as compared to a slurry in which the additive is absent. A slurry in which the additive is absent reaches a temperature of about 4000 to 450xc2x0 Fahrenheit, whereas the slurry having the additive reaches a temperature of about 300xc2x0 Fahrenheit. Decreased temperature of the slurry is beneficial, in part, because high temperatures may destroy some polymers, less insulation is required in the grinding apparatus, heat damage to the grinding apparatus is decreased and the temperature of the grinding operation is better controlled to achieve the desired viscoelastic effects in milling the material.
The additives chemically react with the material to produce a more desirable material particle. The glass transition temperature (Tg) of the material-additive product is reduced as compared to material particle product in which the additive is absent. A lower Tg value is beneficial in producing downstream materials from the material-additive product because a lower temperature is required for the material-additive product to change from a brittle state to a plastic state, which reduces energy costs. Further, the additives chemically react with the material to produce a more tacky material additive particle product as compared to the material particle product in which the additive is absent.
According to an alternative embodiment, the additive is a resin (natural or synthetic). According to other alternative embodiments, the additive is a rosin (i.e., a mixture of monocarboxylic acids having a hydrophenanthrene nucleus) such as gum rosin or wood rosin. Rosins are of particular interest because they tend to act as dispersing agents in the clastomer slurry and effect the tackiness of the material-additive product. According to a preferred embodiment of the present invention, the rosin is tall oil rosin (i.e., a by-product of the paper manufacturing process). According to a particularly preferred embodiment, the tall oil rosin has a low acid number, such as XP56J commercially available from Arizona Chemical Company.
According to an alternative embodiment, the additive is a resin acid (i.e., abietic-type acids that contain an isophropyl group pendent from carbon 13 as numbered using the Chemical Abstracts system, or primaric-type acids that include a methyl and vinyl group pendent from the same carbon atom). According to other alternative embodiments, the resin acid is abietic acid or rosin soap (i.e., rosin leached with sodium hydroxide). The rosin is made water-soluble by neutralizing the rosin with a suitable base, such as ammonia, ammonium hydroxide, an amine (i.e., a Frisch amine). Other additives of interest include any rosin acid, polymerized rosin acid, esters of rosin acid, dispersions of rosin acid, dispersions of esters of rosin acid, copolymers of rosin acid, disproportionated rosin acid, hydrogenated rosin acid, 9-anthracene carboxylic acid, 2-ethylhexanoic acid, acetal R-type acids, or any organic acid that could be soluble in water by neutralizing the acid.
According to an alternative embodiment, the additive oleic acid (i.e., (CH3 (CH2)7CH;CH(CH2)7COOH derived from animal tallow or vegetable oils). Oleic acid may not substantially modify the tackiness of the clastomer-additive product.
According to one embodiment, the additive is an oligimer (i.e., a low molecular weight polymer molecule consisting of only a few monomer units (e.g., dimer, trimer, tetramer)). In another embodiment, the oligimer has a viscosity of about 100,000 CP and tends to act as a dispersing agent in the material slurry. In another embodiment the oligimer is short chain copolymers of styrene and maleic anhydride that offer typical anhydride functionality in a polymeric material such as SMAJ resin commercially available from Sinclair Petrochemicals, Incorporated. According to another embodiment, the oligimer is ethylene-maleic anhydride copolymers such as EMAJ resin commercially available from Monsanto Industrial Chemicals Co. According to an alternative embodiment, the additive is an ester, such as di(2-ethylhexyl) adipate (also known as dioctyl adipate or DOA), DOS, DOD or plasticizers in PVC.
A filler may optionally be added to the upstream slurry or with the supercritical carbon dioxide. The filler may be added to the slurry (i.e., material slurry or material-additive slurry) to combine with the slurry to form an material-additive product or an material product having reinforcing properties, temperature modifying properties, increased high surface area, and/or increased tensile strength. A filler (e.g., nylon) combines with the final material product to give the final product reinforcing properties, temperature modifying properties, increased high surface area, and/or increased tensile strength. According to a particularly preferred embodiment of the present invention, the filler is a nylon material.
The slurry (i.e., material-additive slurry or material slurry) is provided within a grinding apparatus and the material and the additive can be co-ground (or the material is ground) to decrease the material particles to a decreased size. According to one embodiment, the slurry is provided between the two wheels of the grinding apparatus as disclosed in U.S. Pat. No. 5,238,194 issued Aug. 24, 1993 to Rouse et al. for xe2x80x9cMETHOD OF PRODUCING FINE ELASTOMERIC PARTICLESxe2x80x9d and incorporated herein by reference. The grinding stones can be brought into contact with (or spaced a distance apart from) each other. Choosing or maintaining the fixed position of the grinding stones with respect to each other is known in the art of colloid mills. Persons of skill in the art will readily understand how the slurry is input, at a feed pressure, to such grinding stones and how the spacing between the stones is to be adjusted and set. The grinding action generates heat, which may be controlled, modified or reduced by the particular additive in the slurry.
The carrier and the slurry (i.e., material-additive slurry or material slurry) can be separated according to an alternative embodiment, the liquid from the slurry is removed to a capture region positioned outside the outer perimeter of the opposed grinding stones according to a preferred embodiment, the carrier is removed by a centrifuge action, such that the carrier is driven off from the slurry and the material-additive particles or the material particles remain.
The grinding of the slurry (i.e., material-additive slurry or material slurry) may be conducted in a single pass or a multiple pass operation. According to an alternative embodiment, the grinding operation may be conducted in a single pass where the coarse particles that do not pass a screening operation can be feed to a slurry (i.e., material-additive slurry or material slurry) for re-grinding. According to any alternative or preferred embodiment, the grinding operation may be conducted in a multiple pass where two interconnected grinding mills sequentially grind the slurry to a final fine grind state as disclosed in U.S. Pat. No. 5,411,215 issued May 2, 1995 to Rouse for xe2x80x9cTO STAGE GRINDINGxe2x80x9d and incorporated herein by reference. According to the multiple pass grinding operation, a first grinding mill produces an intermediate feedstock of decreased size. The feedstock is re-wet with water or another carrier to form a feed slurry that is then sequentially fed into a second mill. The multiple pass grinding operation, by selecting optimum size of the grinding stones for production of the intermediate feedstock and then for the production of the final minus 80-mesh to 200-mesh product, requires less energy than, for example, the single pass grinding operation.
The material-additive or material product may be dried by heat or air. According to an alternative embodiment, the carrier xe2x80x9cflashes offxe2x80x9d when the ground particles in slurry exit from between the grinding stones to a decreased pressure (i.e., atmospheric pressure). The flash avoids the necessity of having a separate processing step of drying the ground particles, which saves energy costs of running a separator. The material product is recovered.
The material, which preferably chemically bonds with the additive to form the material-additive product, has certain functional characteristics. According to an exemplary embodiment, the material-additive product is a very fine powder having a consistency similar to cooking flour. According to an alternative embodiment, the material-additive product is in the original vulcanized state. The top and bottom variation of the softening point of the rosin-modified material-additive product varies by about 1xc2x0 Celsius. This range of softening variation is advantageous compared to the material product that is not rosin-modified, which has a top and bottom softening point that varies by about 10xc2x0 Celsius.
By using one or more additives with the ground material as described herein, a finely ground product is obtained that is unexpectedly superior in terms of many of its properties, particularly when used in asphalt products such as roofing, paving, building materials, sealants, etc. The final product obtained when using one or more additives as described herein will generally have a bulk density of 26-28 lbs./sq. ft., have a specific gravity from 1.13+/xe2x88x920.02, have 100% of the particles finer than 176 microns or minus 80 mesh. Ground material products prepared as described herein will typically resist phase separation, are storage stable, provide superior low temperature properties and have excellent high temperature stiffness. When the ground material is mixed at a ratio of from 10-15% with asphalt such as WY Sour AC-20, AC-10, Venezu, AC-20, Saudi AC-20 and the like, by any known method, the storage stability of the asphalt product per ASTM D5892 (incorporated herein by reference) also known as the xe2x80x9cCigar Tubexe2x80x9d storage stability test will typically range for eth top (SP, top, xc2x0 F.) from 133-144 and at the bottom (SP, bottom, xc2x0 F.) from 134-156. The change in storage stability between the top and bottom sections generally remains substantially unchanged, that is, the AFxc2x0 from top to bottom will preferably be from 0-2 degrees. This is beneficial, inter alia, because the small change in storage stability of the asphalt will minimize cracking due to temperature changes since the layers of asphalt applied for example on a road, will generally age at the same rate. Asphalt including a ground material with one or more of the instantly described additives will also be easier to store and ship.
The additive can be added in any desired amount. Preferably the additive is added in an amount from 0.05% to 95%, more preferably from 5% to 40% advantageously from 5% to 20% but in a preferred embodiment is added in an amount of about 10% or exactly 10%, in each case being based on the total weight of the material.
It is important to note that the use of the term xe2x80x9cproduction of material particlesxe2x80x9d is not meant as a term of limitation, insofar as any product which may incorporate the material product is intended to be within the scope of the term. For example, the material product may be incorporated into or useful in the production of air blown asphalt, paving membranes, etc., paving cement (i.e., portland cement), the manufacture of any rubber article (e.g., tires, carpet backings, shoe soles, plastic garbage containers, etc.), thermoplastic materials, automotive goods (i.e. underbody coatings), insulation, etc.